8 TOPOGRAPHICAL ANATOMY OF THE 



space by anastomosing with the intercostal branches of the internal 

 thoracic and musculo-phrenic arteries. 



In its course, an intercostal artery furnishes branches to the pleura, 

 ribs, muscles and overlying skin. 



Vv. INTERCOSTALES. — Intercostal veins accompany the intercostal 

 arteries and are to be sought between the artery and the rib. Their 

 termination will be revealed when the interior of the thorax is being 

 examined. 



Nn. INTERCOSTALES. — Intercostal nerves follow the caudal border of 

 the arteries of the same name, and are derived from the thoracic spinal 

 nerves. Eighteen pairs of thoracic nerves (nn. thoracales) leave the 

 vertebral canal by the intervertebral foramina, each nerve receiving a 

 numerical designation in accordance with the thoracic vertebra behind 

 which it leaves the canal. That is, the first thoracic nerve passes 

 between the first and second thoracic vertebrae : the eighteenth thoracic 

 nerve leaves the vertebral canal between the eighteenth thoracic and 

 the first lumbar vertebra. 



Within the intervertebral foramen, or immediately on its exit 

 therefrom, each thoracic nerve divides into a dorsal and a ventral 

 branch. The smaller dorsal branch (ramus dorsalis) crosses the 

 medial border of the levator muscle of the rib, and thereupon divides 

 into a medial and a lateral ramus. The medial ramus runs upwards 

 on the surface of the multifidus and supplies the deep muscles of the 

 back. The lateral ramus passes in a lateral direction under the 

 longissimus dorsi, and has already been noted as piercing the edge of 

 this muscle or emerging between it and the ilio-costalis. It furnishes 

 twigs to these muscles, and ends, as a dorsal cutaneons nerve, in the 

 skin of the back. In the scapular region it supplies the dorsal serratus 

 and rhomboid muscles and ends in a cutaneous nerve in the skin over 

 the scapular cartilage and the ligamentum nuchse. 



The larger ventral branches (rami ventrales) of the thoracic nerves 

 form the intercostal nerves at present under examination. 



The first thoracic nerve contributes to the brachial plexus nearly all 

 the fibres composing its ventral branch. Consequently, the first inter- 

 costal nerve is very small. The second intercostal is also relatively 

 small, because many of the fibres of the ventral branch of the second 

 thoracic nerve go to the brachial plexus. 



The other intercostal nerves are about equal in size but of varying 

 length in accordance with the intercostal space in which they are 

 placed. They accompany the intercostal vessels, lying at first between 

 the external and internal intercostal muscles, and later between the 

 internal muscle and the pleura. The second to the sixth nerves end in 



